Today, many devices for self-administration of liquid drugs are based on technologies for percutaneous delivery, e.g. injection devices or infusion pumps, or for pulmonary delivery, e.g. inhalers. In the art of such drug delivery devices there is a marked distinction between a so-called prefilled device and a so-called reusable device. The term “reusable device” designates a drug delivery device which employs a user exchangeable drug container, whereas the term “prefilled device” designates a drug delivery device which carries a non-exchangeable drug container that is pre-mounted by the manufacturer. Hence, where the former is adapted to be used for exhaustion of multiple drug containers, the latter is meant to be discarded after emptying of the pre-mounted drug container.
In large scale manufacturing and assembly of prefilled drug delivery devices, such as prefilled injection pens commonly used in the diabetes care segment for administration of e.g. insulin or glp-1, the tolerance chains are likely to result in relative positions of specific internal components varying slightly from device to device. For example, when a drug containing cartridge comprising a slidable piston is attached to a pen housing the piston may not perfectly align with a piston actuator in the pen housing adapted to thrust the piston forward in the cartridge for expelling of the drug, establishing an undesired clearance between the piston and the piston actuator which needs to be eliminated by the user performing an initial priming action before the first dose administration.
WO 2014/161952 (Novo Nordisk A/S) discloses an example of a prefilled injection pen. If this pen is produced using conventional techniques an initial clearance between the piston rod foot and the piston may potentially exist when the pen is supplied from the manufacturer, and the user is therefore advised to perform initial priming to ensure that this potential clearance is eliminated before the pen is used for administering a dose to the body. Initial priming consists of the user setting a small dose and activating the delivery mechanism to expel the set dose. The piston rod foot will thereby be urged distally a predetermined distance by the piston rod. The predetermined distance traveled by the piston rod foot will be sufficient to eliminate any initial clearance between the piston rod foot and the piston, but will also cause a small amount of drug to be discharged from the cartridge.
So, not only is initial priming of a prefilled drug delivery device an additional activity which is imposed on the user it also inevitably leads to some wastage of drug. It is therefore highly desirable to provide a prefilled drug delivery device in which the piston and the piston actuator are always in physical contact such that initial priming is unnecessary.
In WO 2014/161952 the piston rod is propelled by a rotatable piston rod guide, which is driven by energy released from a torsion spring. This piston rod guide is coupled to the housing by a unidirectional ratchet preventing rotation of the piston rod guide in a direction which would correspond to a proximal movement of the piston rod. The piston rod is thus prevented from proximal movements relative to the piston, which is a prerequisite for eliminating the risk of a subsequently introduced air gap between the piston rod foot and the piston. However, paradoxically, the requirement of preventing proximal piston rod movements reduces the flexibility in the assembly process, rendering a large scale manufacturing of such prefilled drug delivery devices more vulnerable to the tolerance chains. In the absence of individual adjustment the device assembly process is based on predefined positioning of the respective components relative to the housing, entailing an inherent degree of variability in the end products.